I have a Grohe Integrated Thermostat valve (34 436) that leaks just as it does for many other people. The problem is the middle O ring on the stop valve plunger. I managed to get the O ring sizes from Grohe and find that the middle O ring is an odd size: 6.5 x 3 mm (the other two O rings on the plunger are 8x2 mm, a standard size). I have discovered that such a size is hard to find, but have also discovered that Honda motorcycles use a 6.5x3.1 mm O ring in the cam chain tensioner. I currently have a 7x3 mm O ring in the faucet right now, and it seals (7x2.5 mm does not, nor does 6x3 mm), but it's too big for the valve to operate smoothly. I hope this information helps someone else. It would appear that Grohe has gone out of its way to use odd sizes that force consumers to cough up $35 for an O ring kit (one of the O rings is 32.7x1.3 mm!?) when only two O rings are needed for repair.
Grohe says the two O rings on the stop valve body are 19x1.5 mm and 20x2.5 mm, both readily available.
Should your stop valves leak, remember to get rid of hard water deposits first. I soaked the valve bodies and plungers in a lime-removing liquid to make sure they were clean before replacing the O rings.